In order to produce a protein, a cell transcribes an mRNA from a DNA, i.e., a gene, and further translates the mRNA into the protein. It is clear that if the transcription is suppressed, the protein encoded by the DNA cannot be expressed. Transcription involves many factors called transcription factors. Transcription factors can be classified into basal transcription factors and transcription regulatory factors. While basal transcription factors are involved in most gene transcriptions, transcription regulatory factors are involved in transcriptions during a particular period or under particular conditions. HSF is one of such transcription regulatory factors, and is present in an evolutionarily wide range of species, from yeast to humans. HSF is characterized in that it specifically induces expression of a group of genes controlled thereby although transcription or protein synthesis in general is suppressed under environmental or physical stress such as heat or chemicals such as an amino acid analog, ethanol or a peroxide. It is known that upon activation HSF is turned from a monomer into a homotrimer in the cytoplasm and is transferred into the nucleus so as to recognize and bind to a binding sequence on a DNA called the HSE sequence (a base sequence of about 8 bp or more), thereby inducing and transcribing an mRNA (Mol. Cell. Biol., 13: 2486, 1993). Four types of HSF, HSF1 to HSF4, have been reported. It is believed that HSF1 is induced by external stress such as heat (Mol. Cell. Biol., 17: 469, 1997). Proteins which have the HSE sequence and are transcribed and translated from genes controlled by HSF are generically called heat shock proteins (HSPs), stress proteins, or chaperone proteins. Examples of HSPs include, for example, HSP70, HSP90, HSP100, etc., and it has been reported that HSF actually functions in the expression of these HSPs. Conventionally, flavonoids such as quercetin are known as HSF inhibitors (Cell Struct. Func., 15: 393, 1990).
Stress includes cellular-level stress and animal-level stress, and the relationship between the cellular-level regulation/control of stress and the animal-level treatment of stress is an issue that should be studied. The present invention provides treatment of diseases by controlling the expression of a protein which is induced by stress, i.e., by HSF. Many HSPs are usually synthesized in a cell through a transcription mechanism other than the HSF activation. Some HSPs are believed to be essential for the cell to function properly (Nature, 355: 33, 1992: Nature, 381: 571, 1996). Generally, an inhibitor which provides its function by binding to a protein may, depending on the type of the target protein, affect cells or tissues which are not intended to be treated, thereby causing serious side effects. Since HSF is a stress-induced transcription factor, its inhibitor is believed to inhibit the transcription or the signal transmission during the HSF activation. By nature, HSF is activated to induce expression only when the cell is under a specific environment. The HSF inhibition as used herein aims to affect as little as possible the expression of the essential HSPs which are normally required by the cell while suppressing only the induction by stress, including abnormal induction. Heretofore, there has not been any drug which is capable of specifically suppressing only the induction under stress such as heat, and yet satisfactory in terms of the effectiveness, specificity, or the like.
The HSF activation process, i.e., the HSF modification and signal transmission caused by a stress stimulus such as heat being applied to the cell, has not yet been sufficiently elucidated. If a specific inhibitor is discovered, because of its mechanism it is expected to have a potential to be a new type drug. For example, it is known that a cell acquires a resistance to heat for a certain period of time after a thermotherapy for cancer. This is called acquisition of heat resistance. Since it takes 4-5 days for the heat resistance to disappear, a thermotherapy is typically conducted with a frequency of once or twice per week. It is expected that, if the acquisition of heat resistance can be disabled, the effect of cancer thermotherapy can be improved because it will then be possible to increase the number of times the therapy can be conducted. Moreover, it is clinically widely recognized that the effect of thermotherapy for cancer is greater when it is performed in combination with radiotherapy, chemotherapy, or the like, than when it is performed alone. For the mechanism of acquiring the heat resistance, it is believed that, for example, a protein which has been or is being denatured, is recovered by the action of HSP inducibly produced in a cell by heat, thereby reducing the cell killing effect. However, the details are still unclear. It is believed that HSP prevents apoptosis in a cancer cell. Therefore, it is possible, by suppressing the expression and induction of HSP, to cause normal apoptosis in a cancer cell, thereby controlling immortality or infinite proliferation of the cancer cell.
In the modern world, there has been an increasing number of cases of stress diseases caused by mental stress, including, for example, depression and anxiety. Many of the conventional drugs therefor have not been satisfactory in terms of their effectiveness and side effects. Moreover, the conventional drugs take a long time before they exhibit their effects, and the mechanism of action cannot be explained only with the receptor theory. Under such circumstances, a drug which selectively inhibits reuptake of serotonin has attracted attention as an antidepressant with reduced side effects. On the other hand, a serotonin (5-HT) receptor agonist has also attracted attention as an antianxiety drug. Some of these drugs (e.g., tandospirone citrate) provide both an antidepression action and an antianxiety action. This suggests that depression and anxiety, as stress diseases, are controlled at least partially by a common mechanism. It is expected that the essential characteristics of stress diseases such as depression and anxiety are elucidated, and a new drug are then developed based on a novel action mechanism. One strategy therefor is to associate the cellular-level stress with the animal-level stress from a novel point of view. The present inventors assumed that a drug which is capable of inhibiting activation of a heat shock factor at cellular level, would have an effect on stress diseases such as depression and anxiety at an animal level. Stress and a response thereto have a fundamental structure which is common among hierarchically different levels, i.e., the cell and the animal (Ichiro YAHARA, Stress Protein (ed., Kazuhiro NAGATA) pp. 257-262, 1994). This is supported by following: it is recognized that a stress response is purposive in that exposure to a stress make an animal or cell resistant to the next stress; a stress resistance is similarly obtained through different types of stress; stress is normally transient in an animal or cell, and the like. In view of these characteristics of stress, the present invention aims to treat a disease by controlling an overreaction or uncontrolled reaction in the individual cells, thereby restoring it to a normal state. The inhibition of signal transmission of the heat shock factor activation may be believed to inhibit the stress reaction of a cell and to place the cell in a stress-free state. Therefore, the suppression of stress reaction at a cellular-level through suppression of the HSF activation or the HSP expression can lead to suppression of stress reaction at an animal-level, whereby the inhibition of HSF activity or HSP expression can result in treatment or prevention of stress diseases such as depression and anxiety.
Currently, flavonoids such as quercetin is believed to have suppressive action on the HSP production, and to inhibit the activation of HSF by suppressing the expression of its mRNA. However, the mechanism of the action has not completely been known. Quercetin does not have a sufficient inhibitory effect on the HSF activity for use as a therapeutic. Moreover, there is fear of quercetin exhibiting potential side effects since it has various other physiological activities such as mutagenicity, an antivirus activity, a tyrosine kinase inhibiting activity, a protein kinase C inhibiting activity, and a lactate transport inhibiting activity.
Therefore, it has been desired for a drug to be developed which acts on the HSF activation more specifically so as to have a stronger effect and reduced side effects.